The invention relates to a method for fabricating semiconductor layers.
During the fabrication of thin-film semiconductor components such as thin-film LEDs, for example, it is customary for one or more semiconductor layers to be applied successively to a suitable carrier. The carrier is generally a substrate or a quasi-substrate on which the individual semiconductor layers are successively grown epitaxially. The semiconductor layers, which together constitute the so-called xe2x80x9cuseful layerxe2x80x9d, are then stripped away from the carrier. To that end, a desired separating plane along which the separation is to be effected is provided in the carrier or else in the useful layer.
Before the stripping-away process, the useful layer must be connected to an auxiliary carrier (also referred to as a xe2x80x9csupport waferxe2x80x9d) on account of the small thickness of the layer and the accompanying mechanical instability. Since the surface of the useful layer is generally metallized in order to form electrical terminals, the auxiliary carrier is usually bonded eutectically onto the metallized surface of the useful layer.
However, a composite of a carrier, an useful layer and an auxiliary carrier fabricated in this way tends to split in the metallization plane, i.e. the plane in which the bonding connection between the auxiliary carrier and the useful layer is located, instead of in the desired separating plane. In this case, there is the risk of the useful layer being damaged or partly remaining on the carrier. The useful layer cannot be transferred in this case. On account of the major technological difficulties in the separation of the carrier and the useful layer when using an auxiliary carrier with a metal compound, hitherto the auxiliary carrier has been bonded directly onto the useful layer or adhesively bonded onto glass (xe2x80x9csilicon on glassxe2x80x9d, SOG), so that a metal layer has not been formed between the auxiliary carrier and the useful layer.
It is accordingly an object of the invention to provide a method for fabricating semiconductor layers that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which the semiconductor layer is stripped away from a carrier as far as possible completely along a predetermined desired separating plane. In particular, the intention is to be able to carry out the method with an auxiliary carrier that has a metallic layer or is connected to the semiconductor layer by a metallic layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fabrication method. The method includes providing a carrier, applying a useful layer containing at least one semiconductor layer to the carrier, applying an auxiliary carrier to that side of the useful layer remote from the carrier with a connecting layer disposed between the auxiliary carrier and the useful layer at a joining temperature, and mechanically stripping away the carrier at a temperature greater than or equal to the joining temperature and is less than a melting point of the connecting layer. At least a part of the useful layer together with the auxiliary carrier is stripped away from the carrier.
In the method for fabricating the useful layer containing at least one semiconductor layer, in a first step, the useful layer is applied to the carrier and, in a further step, at a joining temperature, the auxiliary carrier is applied to that side of the useful layer which is remote from the carrier by use of the connecting layer, which preferably has a metallic material. Afterward, the carrier is stripped away at a temperature that is greater than or equal to the joining temperature and is less than the melting point of the connecting layer. The useful layer is separated at least partly, preferably completely, together with the auxiliary carrier from the carrier. The stripping away is preferably effected mechanically.
In this case, the invention is based on the insight that a mechanical strain resulting from the connection of the auxiliary carrier to the useful layer must be reduced to the greatest possible extent in order not to impair the separation process.
Mechanical strains can arise in particular as a result of the temperature change required for connecting the auxiliary carrier to the useful layer. Thus, during the eutectic bonding of the auxiliary carrier, the system is heated to a temperature at which the bonding reaction occurs. The overall system is stress-free at this temperature. The auxiliary carrier is fixed on the useful layer by the formation of the bonding connection. In conventional methods, the composite containing the carrier, the useful layer and the auxiliary carrier is subsequently cooled for further processing.
Since the thermal expansion coefficients of the metals that are customarily used for the bonding connection, such as Au, Pt, Pd, Al, for example, are significantly larger than those of most semiconductors, mechanical stresses arise during cooling, so that the connecting layer is tensile-strained at room temperature.
During the layer transfer that takes place at room temperature in conventional methods, a separating tear is initiated between the carrier and the useful layer. On account of the mechanical strain or the strain field in the system, the separating tear is conducted into the metallization layer and leads to incorrect separation, i.e. to separation along the metal layer and thus to the undesirable stripping away of the auxiliary carrier.
It has been shown in the context of the invention that such incorrect separation can be avoided by a separation at a temperature that approximately corresponds to the joining temperature or is greater than the joining temperature. During mechanical separation at the joining temperature, the separating tear runs between the carrier and the useful layer with the auxiliary carrier in the desired tear plane, i.e. the useful layer is separated together with the auxiliary carrier as desired from the original carrier.
Surprisingly, then, an increase in temperature leads to a better connection between the auxiliary carrier and the useful layer, even though it would be expected in principle that, by way of example, a metal compound becomes softer and thus more unstable in the event of an increase in temperature.
What is essential to the invention is that the composite of the useful layer and the carrier is at least approximately at the joining temperature during the separation process or the composite is heated at least to the level of the joining temperature for the separation process. By virtue of the increase in temperature, the strain gradient in the region of the desired separating plane or thexe2x80x94for example metallicxe2x80x94connecting layer between the useful layer and the auxiliary carrier is compensated for or at least reduced to an extent such that a separating tear, once initiated, propagates in the desired separating plane. To that end, a connection between the useful layer and the auxiliary carrier is used which has a sufficient mechanical stability at the joining temperature and, in particular, does not melt.
A material system whose melting point is sufficiently in excess of the joining temperature after the connection process is preferably used for the connection. Suitable material systems for the connecting layer for the connection of the auxiliary carrier and the useful layer are PdIn, AuSi, PdSi or PtSi, for example. The melting points of such compounds are significantly greater than the respective joining temperatures. Thus, in the case of a PdIn compound, for example, the joining temperature is about 200xc2x0 C., whereas the melting point is 664xc2x0 C. The corresponding temperatures of the other material systems mentioned lie in a similar range.
Materials for the connection in the case of which the difference between the melting point and the joining temperature is greater than or equal to 40% of the joining temperature, in particular greater than or equal to 100% of the joining temperature, are generally preferred in the case of the invention.
In particular, a suitable selection of the joining method and the metallization is essential to the invention, so that mechanically stable connections are present at the joining temperature or similar temperatures. In the case of the invention, the auxiliary carrier and the useful layer are preferably joined together by diffusion bonding or isothermal solidification.
The invention advantageously makes it possible to introduce metallic intermediate layers between the useful layer and the auxiliary carrier that are suitable for a mechanical layer transfer (wafer transfer). As a result, the useful layers which are metallized on the top side, and described in the introduction, can also be processed. The lift-off methods or wedge separation methods known from SOG technology can thus be transferred to material systems in which direct bonding has not been possible heretofore.
In particular, the invention is therefore suitable for semiconductor materials based on nitride compound semiconductors. Nitride compound semiconductors are to be understood as nitride compounds of elements of the third and/or fifth main group of the periodic table, in particular GaN, AlGaN, InGaN, AlInGaN, AlN or InN. Such semiconductor layers are preferably grown (i.e. epitaxially) on carriers containing SiC (silicon carbide), for example in the form of SiC substrates or SiC/Si quasi-substrates.
A quasi-substrate is a composite containing a substrate body, a growth layer and, where appropriate, an intermediate layer disposed in between. Generally, the growth layer is significantly thinner than the substrate body and optimized with regard to an epitaxial deposition of the useful layer. The separation is preferably effected along the intermediate layer. In this case, it is advantageous to modify the bond strength of the intermediate layer such that separation along the intermediate layer is facilitated. By way of example, a silicon oxide intermediate layer is suitable for quasi-substrates with an SiC substrate body and an SiC growth layer.
An Si substrate, for example, can be used as auxiliary carrier.
In addition, the invention enables the (mechanical) layer transfer in the case of heterogeneous useful layer/auxiliary carrier systems, without adhesive bonding being necessary.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating semiconductor layers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.